1. Field of the Invention
The present invention relates generally to bi-level pumping oil and, more particularly, to such systems in which continuous tone images may be displayed by controlling the thresholds at which individual elements in the display are actuated.
2. Description of the Prior Art
Various techniques have heretofore been proposed to display grey-scale or continuous tone images on a bi-level display. These techniques are both interesting and useful, since such displays typically include a matrix of individual, closely spaced cells each of which resides in one of two visual states, either completely energized (ON) or completely de-energized (OFF), and since bi-level displays are particularly well suited for use in systems in which picture data is digitally encoded. Because the panel itself has no inherent capability for representing grey scale, it is the processing of the picture elements and/or the manner in which individual cells are energized that creates a subjective impression in the viewer of continuous tone gradations from very dark to very light. For example, in one prior art processing technique known as ordered dither, the cells of the display panel are divided into a plurality of submatrices of predetermined size such as 16 cells in a four-cell-by-four-cell arrangement. Each cell within each submatrix is assigned a spatially corresponding threshold value from a predetermined "dither matrix" having in this example 16 threshold values distributed between the upper and lower limits in which picture intensity is quantized; numerically successive threshold values are substantially equally spaced throughout the submatrix. In operation, the quantized intensity of each picture element is compared to the threshold thus set for the corresponding display cell. An intensity greater than the assigned threshold value resulting in cell energization, while an intensity less than the threshold maintains the cell in its de-energized condition. Further details of the ordered dither technique are contained in an article by Judice, Jarvis and Ninke entitled "Using Ordered Dither to Display Continuous Tone Pictures on an AC Plasma Panel," Proceedings SID, Fourth Quarter, 1974.
In another prior art technique, the cells of a bi-level display panel are grouped into regions, for example 2 .times. 2 cells wide. The picture intensity corresponding to a particular region is quantized, in this example into one of four possible levels, and the number of energized cells per region is determined by the quantized intensity. Details of this technique are contained in A. Sobel, U.S. Pat. No. 3,647,958 and L. J. Schmersal et al, U.S. Pat. No. 3,845,243.
While both of the aforedescribed techniques, as well as others known in the art, produce satisfactory results for certain types of pictures, the subjective appearance differs from system-to-system and may be better for photographs of people than for graphics, line drawings and text. In addition, certain techniques may not be capable of producing the wide range of tonality needed for a pleasing continuous tone display. Accordingly, it is the broad object of the present invention to provide subjectively pleasing continuous tone images on a bi-level display device, particularly with respect to graphics, line drawings and text. A further object is to achieve a wide range of tonality in the processed picture.